Semiconductor device and method of fabricating semiconductor device

ABSTRACT

To provide a semiconductor device which makes it possible to avoid deterioration in the step coverage property at a gate electrode provided on an operating region and decrease a leakage current between the operating region and the gate electrode. The semiconductor device arranged as a HEMT is made to include an operating region composed of multilayer films, such as a channel layer, an electron supplying layer and other semiconductor layer, and having an island structure independently mesa-isolated from one another. The semiconductor device also includes a gate electrode and an impurity diffusion layer provided on the surface of the operating region, the impurity diffusion layer being doped with an impurity having a conductivity type inverse to the impurity doped into the electron supplying layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation Application of the patentapplication Ser. No. 10/989,359, filed on Nov. 17, 2004, which is aDivisional Application of the patent application Ser. No. 10/339,416,filed on Jan. 10, 2003, which is based on Japanese Priority DocumentJP2002-006946, filed in the Japanese Patent Office on Jan. 16, 2002, theentire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method offabricating the device. More particularly, the present invention relatesto a HEMT (High Electron Mobility Transistor) which intends to reduce agate leakage current.

2. Description of Related Art

FIG. 6 is a plan view showing one example of a semiconductor devicehaving a HEMT structure. FIG. 7 is a cross-sectional view taken along aline VII-VII′ in FIG. 6. As shown in FIGS. 6 and 7, the semiconductordevice has a structure consisting of a semi-insulating substrate 1, abuffer layer 2, a channel layer 3, a spacer layer 4, an electronsupplying layer 5 and a barrier layer 6, which are sequentiallydeposited onto the semi-insulting substrate 1 by using an epitaxialgrowth method (see FIG. 7). Thus, an operating region 11 can be formedas an element of the HEMT.

As shown in FIG. 6, three electrodes, namely, a source electrode 7, agate electrode 8, and a drain electrode 9, are disposed on the barrierlayer 6. The gate electrode 8 is provided so that a Schottky contact isestablished between the gate electrode 8 and the barrier layer 6.Further, the source electrode (pad) 7 and the drain electrode (pad) 9are provided so that an Ohmic contact is established with respect to 5the channel layer 3.

The channel layer 3 is made of an InGaAs layer having no impurity dopedthereinto (i.e., highly purified material). The electron supplying layer5 is made of an n-type InAlAs layer doped with Si as an impurity, forexample.

The semiconductor device (HEMT) constructed as described above has anelectron affinity larger at the channel layer 3 than at the electronsupplying layer 5. Therefore, electrons released from impurity atomsdoped into the electron supplying layer 5 will be moved to the channellayer 3 and intensively concentrated in two dimensions at the surface ofthe channel layer 3. Since the channel layer 3 is made of a highlypurified crystalline material containing no impurity, and hence there islittle dispersion due to the impurity, the electrons concentrated at thesurface of the channel layer 3 can move through the surface thereof witha high electron mobility. Furthermore, since the electron density alsois high, a transistor operating at a high speed can be realized.

The barrier layer 6 is made of a non-doped InAlAs layer. Owing to thepresence of the barrier layer 6, electrons can be prevented from movingbetween the operating region 11 and the gate electrode 8, so that a gateleakage current can be suppressed.

The spacer layer 4 is made of a non-doped InAlAs layer. Owing to thepresence of the spacer layer 4, the channel layer 3 can be protectedfrom an electric influence from impurity ions that have releasedelectrons. Thus, electrons can be moved through the channel layer 3without the influence. The buffer layer 2 also is made of a non-dopedInAlAs layer. Owing to the presence of the buffer layer 2, the channellayer 3 can be protected from an influence from a crystalline defect ofthe semi-insulating substrate 1 made of InP. If the semi-insulatingsubstrate 1 is made of a material having a satisfactory crystallinenature, the buffer layer 2 is not always necessary.

In general, a semiconductor device, as a method for electricallyinsulated separation of adjacent elements, there is employed a method inwhich ions such as B⁺, O⁺ are implanted to create a high resistivityregion between the semiconductor elements requested to be insulated fromeach other. However, if the semiconductor device has a structure inwhich the non-doped InGaAs layer (channel layer 3) and the n-type InAlAslayer (electron supplying layer 5) are epitaxially grown on thesemi-insulating substrate 1 as described above, ion implantation to formthe high resistivity region is impossible. Therefore, if the HEMT hasthe above-described structure, a so-called mesa isolation method isintroduced. That is, a wet etching is effected on the semiconductordevice to remove unnecessary portions so that the operating regions 11can be made into an island structure in which elements are separated andelectrically isolated from one another.

The gate electrode 8 is formed so as to extend from an upper surface ofthe operating region 11, which is separated from other components on thesubstrate as an island structure by means of a mesa isolation method, toa periphery side of the operating region 11 so as to cover a side wall12 a thereof. This method is employed to respond to the following tworequests: a high accuracy in mask alignment at the time when a gateelectrode is formed on the device, and controllability of a draincurrent over the wider area of the operating region 11 owing to theelectric field effect of the gate electrode 8. Meanwhile, the other endside of the gate electrode 8 is similarly extended toward the outside ofthe operating region 11 and connected to a gate leading portion (or apad portion).

However, in the semiconductor device having the above-describedstructure, the gate electrode 8 will be brought into contact with thenon-doped InGaAs layer (channel layer 3) and the n-type InAlAs layer(the electron supplying layer 5) at the side wall portion 12 a of theoperating region 11. Furthermore, the materials of InGaAs and n-typeInAlAs essentially have a narrow band gap, and a Schottky barrierthereof with respect to a gate electrode made of a metal is low.Therefore, there is a problem in which movement of the electrons betweenthe channel layer 3 or the electron supplying layer 5 and the gateelectrode 8, that is, a gate leakage current, is increased so that HEMTperformance is deteriorated.

FIG. 8 shows a conventional arrangement for avoiding the above-describedproblems. As shown in FIG. 8, the operating region 11 is patterned intoan island structure to be mesa isolated. Thereafter, the non-dopedInGaAs layer (the channel layer 3) is selectively etched with respect toother layers made of InAlAs so that the side wall portions of thechannel layer 3 are recessed. Subsequently, the gate electrode 8 isformed. In this way, a space a is provided between the gate electrode 8and the channel layer 3. According to the above-mentioned method,however, the selective etching ratio of the InGaAs layer relative to theInAlAs layer is not satisfactory. Furthermore, it is unavoidable for thegate electrode 8 to be contacted to the n-type InAlAs layer (theelectron supplying layer 5). Therefore, an effect of reducing theaforementioned gate leakage current is not obtained satisfactorily.

FIG. 9 is a diagram for explaining a method in which the gate electrode8 can be prevented from being contacted to a side wall 12 b. As shown inFIG. 9, when the operating region 11 is patterned into an islandstructure to be mesa isolated, the gate electrode 8 is prevented fromcontacting the side wall 12 b by making the side wall 12 b into aninverted-taper shape. In this case, however, the operating region 11cannot have the benefit of satisfactory step coverage at an upper cornerthereof, and the gate electrode 8 will suffer from cut up there.Further, when the operating region 11 undergoes the patterning process,the gate electrode extending direction, is limited depending on thecrystal orientation of the side wall 12 b i.e., a pattern layout islimited because of the inverted-taper shape at the side wall 12 b.

SUMMARY OF THE INVENTION

The present invention seeks to solve the above-identified problems.Therefore, the present invention intends to provide a semiconductordevice and a method of fabricating the device which decreases a leakagecurrent between the operating region and the gate electrode withoutdeteriorating the step coverage property at a gate electrode provided onan operating region in an island structure to be mesa isolated fromother components.

According to the present invention, there is a semiconductor devicehaving an operating region formed by mesa-isolating multilayer films ofsemiconductor layers including a first conductivity type impurity intoan island structure, and an electrode film provided on the operationregion so as to extend from an upper surface to a side wall of theoperating region, wherein the semiconductor device comprises an impuritydiffusion layer including a second conductivity type impurity which isdifferent from the first conductivity type impurity doped into themultilayer films of a surface layer of the side wall of the operatingregion at an area contacting with the electrode film. The secondconductivity type impurity may be an inverse conductivity type relativeto the first conductivity type impurity, and the impurity diffusionlayer including the second conductivity type impurity may be provided ona surface layer of the operating region at an area contacting with theelectrode film. The semiconductor device may be arranged as a HEMTsemiconductor device, and the first conductivity type impurity may be ann-type and the second conductivity type impurity may be a p-type.

According to the present invention, there is proposed a method offabricating a semiconductor device comprising the steps of: formingmultilayer films including a first conductivity type impurity on asubstrate; forming an operating region by processing a mesa-isolationinto an island structure by etching the multilayer films utilizing apattern formed on the multilayer films as a mask; forming, at a sidewall of the operation region, an impurity diffusion layer including asecond conductivity type impurity which is different from said firstconductivity type impurity doped into the multilayer films; and formingan electrode on said operating region from an upper surface to a sidewall after removing said pattern.

In the present invention, there is proposed another method offabricating a semiconductor device comprising the steps of: formingmultilayer films including a first conductivity type impurity on asubstrate; forming an operating region by processing a mesa-isolationinto an island structure by etching the multilayer films utilizing apattern formed on the multilayer films as a mask; forming an isolationfilm on an upper side of the substrate so as to cover the operatingregion after removing the pattern; forming an electrode aperture in saidinsulating film so as to extend from the upper surface to the side wallof the operating region; forming an impurity diffusion layer by applyinga second conductivity type impurity, which is different from the firstconductivity type impurity doped into the multilayer film, to an exposedsurface layer of the operating region through the electrode aperture;and forming an electrode extending from the upper surface to the sidewall of the operating region so that the electrode embeds in theelectrode aperture. The second conductivity type impurity may be aninverse conductivity type relative to the first conductivity typeimpurity, and the impurity of said second conductivity type may be aninverse conductivity type to the first conductivity type impurity. Thesemiconductor device may be arranged as a HEMT semiconductor device, andthe first conductivity type impurity may be an n-type and the secondconductivity type impurity may be a p-type.

According to the above-identified methods of fabricating thesemiconductor device, it is possible to obtain a semiconductor devicehaving the gate electrode provided on the impurity diffusion layerformed on the side wall surface layer of the operating region.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description withreference to the accompanying drawings which illustrate examples of thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A to FIG. 1E are cross-sectional views of a semiconductor deviceunder fabrication steps for explaining a method of fabricating a firstembodiment of the present invention;

FIG. 2A to FIG. 2E are cross-sectional views of a semiconductor deviceunder fabrication steps for explaining a method of fabricating a secondembodiment of the present invention;

FIG. 3 is a plan view of the semiconductor device under a fabricationstep corresponding to FIG. 2D;

FIG. 4 is a cross-sectional view taken along a line IV-IV′ in FIG. 3;

FIG. 5 is a plan view of the semiconductor device under a fabricationstep corresponding to FIG. 2E;

FIG. 6 is a schematic plan view illustrative of a structure of aconventional HEMT;

FIG. 7 is a cross-sectional view taken along a line VII-VII′ in FIG. 6;

FIG. 8 is a cross-sectional view illustrative of a structure of anotherconventional HEMT; and

FIG. 9 is a cross-sectional view illustrative of a structure of stillanother conventional HEMT.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, several embodiments of the present invention will be hereinafterdescribed with reference to the attached drawings. In the followingdescription, steps of fabricating the semiconductor device will beinitially described, and subsequently a structure of the semiconductordevice deriving from the steps of fabrication will be described.Further, similar parts in figures corresponding to those of theconventional semiconductor device described above are identified by thesame reference numerals and a description in detail is omitted.

<First Embodiment>

FIG. 1A to FIG. 1E are cross-sectional views of a semiconductor deviceas the first embodiment of the present invention for explaining a methodof fabricating a HEMT. As shown in the figures, all of the diagrams arecross-sectional views taken along a line which corresponds to the lineof VII-VII′ of FIG. 6 utilized for explaining the conventionaltechnology.

Initially, as shown in FIG. 1A, a semi-insulating substrate (InP) 1 isprepared, and multilayer films consisting of the semiconductor layerscited below are sequentially deposited in order on the semi-insulatingsubstrate 1 to be epitaxial grown. Exemplified thicknesses of each ofthe semiconductor layers are listed below. The semi-insulating substrate1 is not necessarily made of InP but may be made of Si, GaAs or thelike.

The buffer layer(non-doped InAlAs layer) is 2 . . . 500 nm

The channel layer(non-doped InGaAs layer) is 3 . . . 15 nm

The spacer layer (non-doped InAlAs layer) is 4 . . . 2 nm

The electron supplying layer (n-type InAlAs layer) is 5 . . . 10 nm

The barrier layer (non-doped InAlAs layer) is 6 . . . 30 to 100 nm

In particular, it is preferable for the non-doped InGaAs layerconstituting the channel layer to be made up of In_(x)Ga _((1−x)) As(where x represents a number of 0.4 or larger).

Subsequently, a pattern 21 made of an insulating material is formed onthe barrier layer 6. At this time, if the pattern 21 is made of aninsulating film such as SiN or the like, then a lithography technologyis carried out on the insulating film to form a resist pattern. Then,the insulating pattern 21 is formed by etching the insulating film usingthis resist pattern as a mask. On the other hand, if the pattern 21 ismade of a resist material, the pattern 21 is formed by the lithographytechnology.

Then, as shown in FIG. 1B, each semiconductor layers 2 to 6 on thesemi-insulating substrate 1 is patterned by etching using the pattern 21as a mask, so that a part of the multilayer films composed of thesemiconductor layers 2 to 6 is formed into an operating region 11 as anisland structure that is mesa-isolated from the other components on thesemiconductor device. At this time, the etching depth is set in order tocompletely isolated at least the channel layer 3 and the etching reacheseven the buffer layer 2. Further, for this etching, a side etching iscarried out beneath the pattern 21 by isotropic etching, such as wetetching, so that the side wall of the operating region 11 is shaped intoa taper shape.

Thereafter, as shown in FIG. 1C, an impurity diffusion layer 23 isformed on the side wall surface layer of the operating region 11 byimpurity diffusion using the pattern 21 as a mask. The electricconductivity type of the impurity diffusion layer 23 is set to a p-typewhich is inverse to that of the electron supplying layer 5 made of ann-type InAlAs layer. In this case, Zn is selected as a p-type impurity,and the diffusion processing is carried out under a diethyl-Znatmosphere at a temperature of about 60° C. When this diffusionprocessing is carried out, the impurity diffusion layer 23 can be formedover the entire exposed area of the surface layer except the coveredarea with the pattern 21.

In the next step, as shown in FIG. 1D, by an anisotropic etching usingthe pattern 21 as a mask, the impurity diffusion layer 23 is partly leftat the side wall portion of the operating region 11, and the impuritydiffusion layer 23 formed at the other portion (surface layer of thebuffer layer 2) is completely removed. This anisotropic etching iscarried out by an RIE (reactive ion etching), an ion milling or thelike. After the etching is completed, the pattern 21 is removed.

Subsequently, as shown in FIG. 1E, a gate electrode 8 is provided bymeans of vapor deposition or the like so as to cover from the upperportion to the side wall of the operating region 11 and also to extendtoward the outside of the same. The gate electrode 8 is composed of, forexample, an underlayer made of Ti (50 nm), an intermediate layer of Pt(50 nm) and an upper layer of Au (300 nm), sequentially deposited fromthe lower to the upper thereof in this order. These films may bedeposited on the upper surface of the device, and, thereafter, a patternetching is carried out to form the shape of the gate electrode 8.Alternatively, the pattern formation may be carried out by a knownmethod, such as a lift-off method.

After the above processes are completed, although not shown, a sourceelectrode and a drain electrode brought into ohmic contact with thechannel layer 3 are formed on the barrier layer 6. These electrodes maybe formed by depositing, for example, patterning films of Ni (40 nm) onan AuGe film (160 nm), the film to form the shapes of the electrodes. Ifnecessary, a heat treatment (e.g., a heat treatment at a temperature of400° C.) may be carried out after the formation of the electrodes sothat an Ohmic region is created directly beneath the source electrodeand the drain electrode. In this way, resistance between the channellayer 3 and the source electrode and resistance between the channellayer 3 and the drain electrode can be further decreased.

The HEMT 25 obtained by the above processes comes to have the followingfeatures. That is, a Schottky contact is established between the barrierlayer 6 constituting the upper surface of the operating region 11 andthe gate electrode 8.

Therefore, the HEMT 25 is equipped with a Schottky-type gate electrode.This HEMT 25 can be operated by controlling the electric current flowingbetween the source electrode and the drain electrode (drain current),since the gate voltage can change the thickness of a depletion layerunder the gate electrode 8.

In particular, according to the above arrangement of the HEMT 25, theoperating region 11 is covered at its side wall surface layer with theimpurity diffusion layer 23 having doped therewith the impurity of whichan inverse conductivity type (p-type) to that of the impurity, i.e.,n-type, doped into the electron supplying layer 5, and the gateelectrode 8 is provided on the impurity diffusion layer 23 at the sidewall portion of the operating region 11. For this reason, a PN junctionis established at the contact face between the channel layer 3 and thegate electrode 8 and between the electron supplying layer 5 and the gateelectrode 8. Therefore, as compared with a conventional HEMT in whichthere is no impurity diffusion layer 23 provided and the contact facebetween the channel layer 3 and the gate electrode 8 and between theelectron supplying layer 5 and the gate electrode 8 are not the PNjunction but a Schottky contact, the HEMT according to the presentinvention will have a relatively higher energy barrier between thechannel layer 3 and the gate electrode 8 and between the electronsupplying layer 5 and the gate electrode 8. As a result, carriermobility can be suppressed between the gate electrode 8 and theoperating region 11 at the side wall portion of the operating region 11,with the result that the gate leakage current can be decreased.

With the above arrangement, the gate electrode 8 will become moretolerable in being applied with a reverse-voltage. Therefore, a circuitemploying the HEMT will have a wider operating margin. Moreover, noisecaused by the gate leakage current can be more suppressed (i.e., a noisefactor NF can be decreased).

Moreover, according to the above arrangement of the HEMT, the abovementioned advantages can be obtained without shaping the side wall ofthe operating region 11 into an inverted-taper. Therefore, the stepcoverage property can be satisfactorily maintained at a shoulder-shapedportion of the operating region 11, so it is possible to prevent thegate electrode 8 from being cut at the comer portion and formed properlyon the operating region 11.

<Second Embodiment>

FIG. 2A to FIG. 2E are cross-sectional views of a semiconductor deviceas the second embodiment of the present invention for explaining amethod of fabricating a HEMT. As shown in the figures, all the diagramsare cross-sectional views taken from a direction which corresponds tothe line of VII-VII′ of FIG. 6 utilized for explaining the conventionaltechnology.

Initially, the fabrication steps shown in FIGS. 2A and 2B are carriedout in a manner similar to those of the first embodiment described withreference to FIGS. 1A and 1B, and after forming the operating region 11as an island structure that is mesa-isolated from other components onthe semiconductor device, the pattern 21 utilized as a mask is removed.The pattern 21 may be formed of an insulating film, such as of SiN, or aresist material.

Subsequently, as shown in FIG. 2C, an insulating film 31 made of SiN orthe like and having a thickness of 200 nm is formed over thesemi-insulating substrate 1 so that this insulating film 31 covers thewhole area of the operating region 11.

Then, as shown in FIG. 2D, a gate aperture 31 a is formed in theinsulating film 31. This gate aperture 31 a is formed so as to expose anarea at which the gate electrode, which is formed in the subsequentfabrication step, will contact the operating region 11.

Thereafter, an impurity diffusion process is carried out through thegate aperture 31 a on the surface of the operating region 11 so that animpurity of an inverse conductivity type (i.e., a p-type) to that of theelectron supplying layer 5 made of an n-type InAlAs layer is applied.Thus, an impurity diffusion layer 33 of the p-type is formed on theoperating region 11. The impurity diffusion layer 33 is formed in amanner similar to that of the first embodiment that is described withreference to FIG. 1C.

FIG. 3 is a plan view of the device under the fabrication step shown inFIG. 2D. In other words, FIG. 2D is a cross-sectional view taken along aline II-II′ in FIG. 3. FIG. 4 is a cross-sectional view taken along aline IV-IV′ in FIG. 3. As shown in these figures, the gate aperture 31 aformed in the insulating film 31 covering the operating region 11 isformed so as to correspond to a portion of the operating region 11 onwhich the gate electrode is to be formed in the subsequent fabricationstep. Therefore, the impurity diffusion layer 33 formed by the impuritydiffusion processing carried out through the gate aperture 31 a isprovided so as to extend across the upper surface of the operatingregion 11 from one side wall to the other side wall.

After completing the above-mentioned fabrication steps, as shown in FIG.2E, the gate electrode 8 is formed so as to embed the inner space of thegate aperture 31 a. The gate electrode 8 is formed in a manner similarto that of the first embodiment.

FIG. 5 is a cross-sectional view taken along a line whose direction isidentical to that of the line IV-IV′ in FIG. 3 but the semiconductordevice under a fabrication step as the target of illustration is thatshown in FIG. 2E. As shown in FIG. 5, after the gate electrode 8 isformed so as to embed the inner space of the gate aperture 31 VII-VII′,source aperture 31 b and a drain aperture 31 c are formed to reach theoperating region 11 in the insulating film 31 at both the side portionsof the gate aperture 31 a. Then, similarly to the first embodiment, asource electrode 7 and a drain electrode 9 are formed so as to embed theinner space of the source aperture 31 b and the drain aperture 31 c,respectively. In this case, ohmic contact is established between thechannel layer 3 and the source electrode 7 and between the channel layer3 and the drain electrode 9.

If necessary, a heat treatment (e.g., a heat treatment at a temperatureof 400° C.) may be carried out after the formation of the electrodes, sothat an Ohmic region is created directly beneath the source electrode 7and the drain electrode 9. In this way, resistance between the channellayer 3 and the source electrode 7 and resistance between the channellayer 3 and the drain electrode 9 can be further decreased.

The HEMT 35 obtained by the above processes comes to have the followingfeatures. That is, the gate aperture 31 a is provided so as to extendfrom the upper surface to the side wall of the operating region 11, andthe impurity diffusion layer 33 having an electric conductivity ofp-type is formed at the bottom of the gate aperture 31 a. Further, sincethe gate electrode 8 is formed within the gate aperture 31 a, the gateelectrode 8 can serve as a junction-type gate electrode. This HEMT 35can be operated by controlling an electric current (drain current)flowing between the source electrode and the drain electrode, since thegate voltage can change the thickness of a depletion layer under thegate electrode 8.

In particular, according to the above arrangement of the HEMT 35, theoperating region 11 is covered at its surface layer with the impuritydiffusion layer 33, the impurity diffusion layer 33 is doped with animpurity having a conductivity type (p-type) inverse to that of theimpurity, i.e., an n-type, which is doped into the electron supplyinglayer 5, and the gate electrode 8 is provided on the impurity diffusionlayer 33 at the operating region 11. For this reason, similar to theHEMT 25 of the first embodiment, a PN junction is established at thecontact face between the channel layer 3 and the gate electrode 8 andbetween the electron supplying layer 5 and the gate electrode 8.Accordingly, it becomes possible to obtain advantages similar to thoseof the first embodiment.

Furthermore, according to the above-described methods of fabricating thesemiconductor device, the above-mentioned HEMT 35 can be obtained byadding only one fabrication step, i.e., a step of diffusing an impurityprocess, to a series of steps of fabricating the semiconductor device asthe first embodiment. Therefore, a rise in the production cost of thesemiconductor device can be reduced.

The above-described embodiments are merely examples of the presentinvention, and, therefore, the present invention is not limited to theabove-described embodiments. Therefore, the present invention can beapplied similarly to a case where the operating region is composed ofsemiconductor layers different from those of the above-described examplematerials, the multilayer structure or the like, so long as thesemiconductor device has an operating region which is composed of aplurality of multilayer films made of semiconductor layers and ismesa-isolated from other components in an island structure on thesemiconductor device. Also, such a semiconductor device applied with thepresent invention will produce the same advantages as those of theabove-described embodiments.

1-10. (canceled)
 11. A semiconductor device comprising: a substrate; abuffer layer over said substrate; a channel layer over said bufferlayer, said buffer layer being between said channel layer and saidsubstrate; a spacer layer over said channel layer, said channel layerbeing between said spacer layer and said buffer layer; an electronsupplying layer over said spacer layer, said spacer layer being betweensaid electron supplying layer and said channel layer, said electronsupplying layer having a first conductivity impurity; a barrier layerover said electron supplying layer, said electron supplying layer beingbetween said barrier layer and said spacer layer; an impurity diffusionlayer in contact with said buffer layer, said channel layer, said spacerlayer, electron supplying layer and said barrier layer, wherein saidimpurity diffusion layer includes a second conductivity impurity inversein conductivity relative to said first conductivity impurity, whereinsaid first conductivity type impurity is a p-type and said secondconductivity type impurity is an n-type, and wherein said substrate is asemi-insulating substrate (InP).